1. Field of the Invention
The present invention relates to a lens barrel provided in an imaging apparatus (such as a digital still camera and a digital video camcorder), a lens barrel used in an exchangeable lens, and an imaging apparatus. The present invention particularly relates to a technology for achieving automatic functions using a coreless stepper motor and manual functions that allow manual operation.
2. Description of the Related Art
There has been a camera including a lens barrel using an actuator to drive a lens holding frame that holds a lens. To achieve automatic functions (such as an autofocus function and an automatic zoom function) of the camera, a DC motor, an ultrasonic motor, or any other suitable motor is used as the actuator to drive the lens holding frame.
Although a DC motor is characterized by its inexpensiveness, a gear-based reduction mechanism is typically required to provide a driving torque necessary to move the lens along the optical axis thereof. The gear-based reduction mechanism produces gear noise when the autofocus function or the automatic zoom function is performed, disadvantageously resulting in degradation in quality of the lens barrel. Further, when a DC motor is used, a photo-interrupter or any other suitable position detection sensor is necessary to stop the motion of the lens with precision, which is typically required to achieve the optical performance of the lens barrel. Moreover, incorporating the DC motor and the reduction mechanism in the lens barrel may result in not only a rugged shape and hence unpleasant exterior, but also increase in size of the lens barrel.
On the other hand, when an ultrasonic motor is used, the degradation in quality due to driving noise (gear noise) will not occur because an ultrasonic motor is driven at a driving frequency outside the human audible range. However, since the conditions under which an ultrasonic motor is assembled in a lens barrel tend to vary, unusual noise disadvantageously tends to occur when the ultrasonic motor is driven. Further, since drive control of an ultrasonic motor involves considering the relationship between the nonlinear driving frequency and the voltage, a lens barrel using an ultrasonic motor has a variety of problems, such as increase in the number of steps necessary for design, repair, and restoration, resulting in increase in cost.
To address the above problems, an electromagnetic conversion-type coreless motor has been used, instead of a DC motor or an ultrasonic motor. Specifically, in this technology, a cylindrical field magnet is secured around the outer circumferential surface of a movable lens barrel that rotates relative to a fixed lens barrel, and an armature coil is secured in the fixed lens barrel, which faces the field magnet. A magnetic sensing element is disposed on a conductor that contributes to the torque produced by the armature coil, which faces the field magnet (See JP-A-59-94709, for example).
In recent years, consumers have placed greater importance, when it comes to a higher-performance zoom lens, on whether or not the lens has manual functions. In particular, a lens barrel used in an exchangeable lens needs to allow a photographing user to arbitrarily choose between automatic functions (such as an autofocus function and an automatic zoom function) and manual functions (such as a manual focus function and a manual zoom function).
In another known technology, the operation mode is switched between an autofocus operation and a manual focus operation. Specifically, the technology relates to a lens barrel including a vibration wave motor that produces a driving force for the autofocus operation, a gear train that transfers the driving force produced by the vibration wave motor, and a gear member that switches the operation mode between the autofocus operation and the manual focus operation (see JP-A-2006-259130, for example).